In Japanese Patent Publication Tokkai-Hei-7-144119, a three-way catalyst and a hydrocarbon (hereafter HC) catalyst are disposed in series in an exhaust passage. The HC trap catalyst is disposed downstream of the three-way catalyst.
In a cold operating condition, that is to say, in a condition in which the three-way catalyst is not activated, HC discharge from the engine is trapped in the HC trap catalyst.
After the temperature of the HC trap catalyst reaches a release temperature, the HC trap catalyst begins releasting HC. Released HC is oxidized in the HC trap catalyst.
In Japanese Patent Publication Tokkai-Hei-6-81637 and Tokkai-Hei-5-10178, and HC trapper and a three-way catalyst are disposed in an exhaust passage. HC released from the HC trapper is oxidized in the three-way catalyst disposed downstream of the HC trapper.
While the HC trapper is releasing HC, a control unit for controlling the operation condition of the engine controls an air-fuel ratio to a lean air-fuel ratio so that the exhaust gas flowing into the three-way catalyst creates an oxidizing atmosphere.
In order to accuately control the lean air-fuel ratio with respect to the amount of released HC, in the device disclosed in Tokkai-Hei-6-81637, the control unit controls the lean level according to the duration of the HC release.
In the device disclosed in Tokkai-Hei-5-10178, an oxygen concentration sensor is disposed downstream of the three-way catalyst and the lean level is controlled according to the oxygen concentration that is detected by the sensor.